This invention relates generally to controlling a speed of a vehicle and, more particularly, to methods and a system for time of arrival control of a vehicle using available speed range.
Although aircraft have historically been controlled only in the lateral and vertical domains, the use of aircraft control in the time domain may enable advanced airspace management resulting in increased capacity. The use of time-based arrival management facilitates earlier landing time assignments and more efficient use of the runway. The use of time-based arrival management results in economic benefits if each aircraft can determine its desired landing time using its most fuel optimum flight profile.
The ability to control aircraft in the time domain is enabled primarily by the use of a Required Time-Of-Arrival (RTA) constraint on a waypoint in the flight plan. In addition to controlling to the Required Time-of-Arrival, the algorithm computes an estimated Earliest and Latest Time-of-Arrival using the maximum and minimum operating speeds, respectively. However, the RTA algorithm was designed with enroute operations in mind and has drawbacks when used for operations in the terminal phase.
Important parameters associated with RTA operation as used herein are:                (1) RTA waypoint (crew entered or uplinked)                    The waypoint where a required crossing time is specified.                        (2) RTA time (crew entered or uplinked)                    The required crossing time expressed in hours:minutes:seconds (traditionally in GMT, but not limited to only in GMT).                        (3) RTA tolerance (default, crew entered, or uplinked)                    The allowable plus and minus crossing time tolerance that is considered to be on-time expressed in seconds.                        (4) Current ETA (computed)                    Estimated time of arrival at the RTA waypoint.                        (5) First time (computed)                    Earliest possible time of arrival using the fastest allowable speed within aircraft limits.                        (6) Last time (computed)                    Latest possible time of arrival using the slowest allowable speed within aircraft limits.                        (7) Distance to RTA waypoint (computed)                    Along path distance to go to the RTA waypoint.                        (8) RTA Error (computed)                    Difference between (2) RTA time and (4) Current ETA expressed as EARLY or LATE time in hours, minutes and seconds when the difference is outside (3) RTA tolerance.                        
Upon entering the RTA waypoint (1), the operator is prompted with RTA time (2) equal to the predicted ETA using the default cost-optimal flight profile. This is the desired time of arrival using minimum cost profile for the flight. The operator can change the prompted value by entering a new value that may be assigned by air traffic control. The resulting RTA speed target shown is provided as the active speed command to the autopilot and displayed on the primary flight display. The target speed may be overridden by any applicable speed restriction. The restricted speed is taken into account when computing the estimated time of arrival. By following the active speed command, the aircraft should achieve the RTA if it is within the aircraft speed bounds to do so. In at least one known implementation, a time profile comprising the ETA at each waypoint and trajectory change point is computed using the current assumed speed profile (using the current cost index). The earliest and latest possible times-of-arrival are estimated only for the RTA waypoint, using the maximum and minimum cost index, respectively. However, improvements to the RTA algorithm for operations constrained late in descent including at the runway threshold itself are needed.